The present invention relates to a dot line printer which carries out printing on a print sheet while shuttling a hammer bank back and forth in a direction transverse to the print paper.
Dot line printers include a hammer bank wherein a plurality of dot print hammers are juxtaposed along a print line. Each print hammer is made up of an elongated leaf spring having one end to which a dot pin is attached and another end secured to a mounting plate. The upper portion of the print hammer is held in a retracted position by a magnetic attraction of a permanent magnet and is fired or released therefrom by supplying a pulsating current to an electromagnetic coil wound around the pole of a rear yoke, thereby making a dot impression on a print paper through an ink ribbon.
In such dot line printers, it is not desirable to simultaneously fire two adjacent print hammers. Because, in order for adjacent print hammers to be fired simultaneously, it is necessary that the duration of the pulsating currents, supplied to the electromagnetic coils, be prolonged and/or the peak levels thereof be raised. Japanese Patent Publication (Kokoku) No. 55-10385 discloses a printer of the type wherein print hammers impinge against print types (characters and symbols) embossed on a rotating drum to thus print characters and/or symbols on a print paper, in which it is proposed to displace the positions of the print types in the circumferential direction of the drum from their home positions. By the displacement of the print type positions, the adjacent hammers are not fired simultaneously.
With respect to the dot line printers to which the present invention pertains, U.S. Pat. No. 4,386,563 (corresponding to DE-OS 3,223,274) and U.S. Pat. No. 4,550,659 disclose dividing the dot print hammers into two groups and displacing the respective hammer positions of the second group by a distance corresponding to a half of a dot from their home positions. A half of a dot is herein defined by a half of a reciprocal of a print density which is defined by a number of dots per a unit length (inch). With such arrangement of the hammers, the number of the hammers which are fired simultaneously is reduced. Undue electric power consumption is avoided and the peak level of the pulsating current does not need to be raised, thereby allowing to use a compact-size power supply unit.
However, such technique is not effective for high speed dot line printers having a printing capability of, for example, about 500 lines per minute for the printing of Japanese Kanji characters. To increase the printing speed, an increased number of the hammers must be provided. The number of the simultaneously fired hammers is, however, increased as the number of the hammers is increased. In the case of 300 hammers, half of them may possibly be fired simultaneously. Hence, the electric power consumption is increased because of the increase of the simultaneously fired hammers.
Further, if dot line printers are designed so that the second half of the hammers displaced by a half of the dot from their home positions are allowed to be fired simultaneously with the firing of the first half of the hammers, the dot impressions are further made by the second half of the hammers in positions between two successive dots impressed by the first half of the hammers. Although high quality printing can be accomplished, an increase of the electric power consumption results.